An automotive carburetor gasket seals a fluid passage between the relatively hot manifold and the relatively cool carburetor. In compliance with recent governmental regulations, automobiles are now being designed to operate at hotter manifold temperatures; and this requires improved insulation of the carburetor in order to reduce fuel bowl temperatures. Although prior manifold-carburetor gaskets provide the necessary sealing, they do not provide the desired heat insulation, and they display poor durability at the high temperatures now being encountered. It is expected that over the next several years the required heat insulating capabilities of carburetor gaskets will become increasingly more stringent.
In U.S. Pat. No. 3,986,721, which is commonly owned with this application, there is disclosed a carburetor gasket wherein the sealing function is provided by a single compressible gasket element which is cut from a sheet or slab of fibrous insulating material such as asbestosboard, vegetable fiberboard, cork composite, or the like. The fluid passage through which the gasoline-vapor air mixture flows from the manifold to the carburetor is encircled by this element. The compressible element is in turn surrounded by and bonded to a rigid (incompressible) rim of phenolic resin or the like. The rim has a thickness slightly less than the thickness of the compressible gasket element so that the latter stands proud of the rim by the amount by which the gasket is to be compressed in mounting, the rim thus functioning as a compression limiting stop to prevent over-compression of the gasket. Bolt holes formed at least partly in the rim are provided for mounting the gasket between the manifold and the carburetor flange.
Gaskets of that type have effectively met previous heat insulation requirements for automotive use. However, in order to meet current and expected future requirements, the need exists for a gasket having improved (i.e., lower) heat transfer characteristics. Moreover, if gaskets of the type shown in patent No. 3,986,721, previously identified, are to be used at high temperature conditions, a compromise as to choice of gasket materials is inevitably necessary. Cellulose (vegetable) type fiberboard has good heat transfer characteristics (it transfers heat at a relatively low rate), and this favors its selection as the material for the compressible element. However, it is relatively poor in its resistance to degradation at high service temperatures (above about 250.degree.-275.degree. F.), which tends to shorten its life in installations where use temperatures are high. On the other hand, asbestosboard offers very good heat resistant qualities, i.e., it can withstand exposure to higher temperatures for longer times than the cellulosic board, but its heat transfer rate is not as low. Thus, the choice of either carries some disadvantages, along with its advantages.